1. Field of the Invention
The present invention relates to a planar light source unit used as a backlight for illuminating a display such as a liquid crystal display (LCD) panel from back surface thereof.
2. Description of Related Art
There have been known backlight devices comprising planar light source units as means for illuminating LCD panel or the like used in mobile terminal devices, laptop type computers or the like.
This kind of planar light source unit has a structure as shown in FIGS. 22A to 22C (see, for reference, Japanese Patent Laid-Open 2003-337333, FIG. 17).
As shown in FIGS. 22A and 22B, the planar light source unit 120 includes a light guiding plate 101 having upper and lower surfaces 101a and 101b, LEDs 102 acting as light emitting sources, a diffusing plate 103, a Py prismatic sheet 104, a Px prismatic sheet 105, and a reflective plate 106. Here, reference number 107 shows a transmissive or semi-transmissive type LCD panel to be illuminated by the planar light source unit 120.
The LEDs 102 are held on an LED substrate 102b to face an incident surface 101c of the light guiding plate 101, and the diffusing plate 103, the Py prismatic sheet 104, and the Px prismatic sheet 105 are disposed to be laminated on the light guiding plate 101 sequentially. The reflective plate 106 is disposed close to and to face the lower surface 101b of the light guiding plate 101.
Light emitted from the LEDs enters the light guiding plate 101 through the incident surface 101c. The entered light is transmitted within the light guiding plate 101 while undergoing repeated reflection between the upper and lower surfaces 101a and 101b and emitted from the upper surface 101a during this repeated reflection. The lower surface 101b is formed from a finely corrugated diffused reflection surface or prisms. The internal light in the light guiding plate 101 is partly reflected on the diffused reflection lower surface 101b to be directed to the upper surface 101a and partly emitted from the diffused reflection lower surface 101b to be directed to the reflective plate 106 by reflection and refraction of light. The reflective plate 106 reflects the light emitted downwardly by refraction through the lower surface 101b to return the reflected light to the interior portion of the light guiding plate 101, thereby enhancing utilization efficiency of light.
The light emitted from the upper surface 101a of the light guiding plate 101 reaches the diffusing plate 103, and a direction of the light is adjusted to some extent toward the LCD panel. Furthermore, light having an angle toward y-axial direction (see coordinate axes x, y and z in FIG. 22A) is adjusted by the Py prismatic sheet 104, light having an angle toward x-axial direction is adjusted by the Px prismatic sheet 105, and light emitted from the Px prismatic sheet 105 is finally aligned in generally parallel to z-axial direction. The light aligned parallel to z-axial direction enters the LCD panel 107 to provide an ideal state of light passing through the LCD panel, whereby enabling a clear display having a high SN (signal-noise) ratio.
However, the above-mentioned planar light source unit has problems as follows.
Because light is reflected in all directions on the lower surface 101b, much light enters the upper surface 101a at an angle near to a critical angle as shown in FIG. 22C. The light refracts at an angle which is close to 90° to a normal line of the upper surface 101a, in other words at an angle which is close to the horizontal direction. In this case, the light may not reach the diffusing plate 103, or, even if it does reach, because of a large incident angle, it is difficult to change the directions of light and to direct light from the diffusing plate 103 into the Py prismatic sheet 104. In this way, on the whole, there is a problem that it is difficult to adjust the directions of the light emitted from the LEDs 102 and to direct the light for illuminating a display effectively.
Therefore, to improve this problem, a planar light source unit using a light guiding plate 101 provided with an optically anisotropic diffusing surface 101h generating hologram diffusion or hairline diffusion has been used. In the planar light source unit, a plurality of prisms are provided on the lower surface 101b of the light guiding plate 101. In other respects, the structure is generally the same as for the planar light source unit 120 shown in FIG. 22A. This kind of technology uses, for example, a known principle shown in page 5 of the specification of U.S. Pat. No. 6,347,873 B1.
More specifically, an anisotropic diffusing surface 101h similar to a predetermined hologram is formed on the upper surface 101a of the light guiding plate 101 (including a case that an incident angle of light to the anisotropic diffusing surface 101h is near to a critical angle) to allow cone-shaped diffusion light φ01 having a desired angle other than the exit angle of 90° (horizontal direction) to be emitted from the anisotropic diffusing surface 101h. In this way, it is possible to improve on the problem caused when a large incident angle is set relative to the diffusing plate 103 shown in FIG. 22C, increase the utilization efficiency of light entering the diffusing plate 103, and increase brightness of illuminating rays.
To briefly state the principle, the diffusion surface 101h is, for example, formed into hologram, to reproduce diffusion light rays which is object light having incident angles within a predetermined angular width when internal light rays having incident angles within a predetermined angular width enter the light guiding plate as reference light rays. The internal light is emitted upward from the light guiding plate 101 as diffusion light rays φ01 having incident angles within a predetermined angular width range by being reflected on one or more times on prisms provided on the lower surface 101b. 
Here, the predetermined incident angular width to be diffusion light rays φ01 depends on a surface state. Even if the surface is formed into a concave state, convex state, or whatever of the diffusing surface, if the surface state is constant, the incident angular width of light rays is also constant regardless of the position of the surface.
Here, the anisotropic diffusion light is emitted with diffusion larger in a width direction (y-axial direction) perpendicular to a sectioned direction (B-B direction) in FIG. 23B than the sectioned direction in a sectional plane of the diffusion light φ01, as shown in FIG. 23C. In other words, the diffusing surface 101h has an anisotropic diffusing surface having high diffusion property in the width direction (y-axial direction).
On the other hand, there is an internal light deficiency area on the upper surface of the light guiding plate 101 corresponding to the space between adjacent LEDs 102 and positioned near the LEDs 102, to which the internal light (light emitted from the LEDs 102, entering the light guiding plate and transmitted in the light guiding plate) does not reach and no light is emitted directly from this internal light deficiency area.
Therefore, it is required that anisotropic diffusion light having a larger diffusion in the width direction (the y-axial direction parallel to the incident surface 101c) is emitted from the diffusing surface 101h in an area (light emitting area) adjacent to the internal light deficiency area.
However, even in the planar light source unit improved in this way, the following problems arise.
In the conventional light guiding plate 101 having the upper surface provided with the anisotropic diffusing surface 101h, the anisotropic property and the intensity of diffusion light are uniform throughout the whole of the light guiding plate. In other words, diffusion occurs with the same anisotropic diffusion light intensity, whether it be in a position close to the LEDs 102 or in a position remote from the LEDs 102.
By the way, in the case of such diffusion light, as the angular width of the diffusion increases, the portion of the emitted light for which the exit angle is large increases and the conversion efficiency of the entire emitted light in a perpendicular direction is reduced. That is to say, the anisotropic diffusion light intensity and the final conversion efficiency to illumination light are closely related. Therefore, in this case, the conversion efficiency is constant regardless of the place within the light guiding plate. Here, the intensity of the internal light in the light guiding plate 101 tends to be reduced with increasing distance from the LEDs 102 because a light path length increases, a solid angle decreases, and diffusion light is emitted along the way.
On the other hand, intensity or brightness of the illumination light depends on the product of the intensity of the internal light and the conversion efficiency. Therefore, the illumination light is significantly increased in a central portion of the light guiding plate 101 and a portion of the light guiding plate opposite the LEDs 102, resulting in uneven brightness of the illumination light. This occurs for the following reason.
In the central portion of the light guiding plate 101 and the portion of the light guiding plate opposite the LEDs 102, even though there are no substantial internal light deficiency areas due to the spaces between the LEDs, because the anisotropic diffusion light intensity on the anisotropic diffusing surface 101h is even throughout the entire light guiding plate, the anisotropic intensity unduly increases in the internal light deficiency areas.